LUN-1446: add default icon for plugin just in case easy_thumbnails decides to throw InvalidImageFormatError.
This is required for the blog migration since we're moving plugins around. Even if the image is not valid plugin data should be migrated.
Provided default image icon for image plugin even if thumbnails cannot get generated.
SHould not throw 500 if filer image was trashed.
Merge pull request #39 from pbs/LUN-1446

Provide default icon if fetching the thumbnail fails